This also
- merges basic/linux and shared/linux,
- moves BPF_JUMP_A() to basic/missing_bpf.h,
- copies from usrspace kernel headers directory generated by 'make headers',
rather than copying from kernel tree,
- copies const.h into our tree to reduce change in ethtool.h,
- copies auto_fs.h into our tree to reduce change in auto_dev-ioctl.h.
IPPROTO_L2TP was moved from linux/l2tp.h to linux/in.h [0], so let's
reflect that change to fix build with newer kernels:
```
In file included from ../src/libsystemd/sd-netlink/netlink-types-genl.c:10:
../src/basic/linux/l2tp.h:16: error: "IPPROTO_L2TP" redefined [-Werror]
16 | #define IPPROTO_L2TP 115
|
In file included from ../src/libsystemd/sd-netlink/netlink-types-genl.c:3:
/usr/include/netinet/in.h:85: note: this is the location of the previous definition
85 | #define IPPROTO_L2TP IPPROTO_L2TP
|
cc1: all warnings being treated as errors
```
When at it, update the rest of the headers we ship as well.
[0] 65b32f801b
